In a conventional earth moving machine having an earth digging implement mounted on one end, usually the rear end, the implement, hereinafter referred to as the backhoe, is mounted on a swing post or mast that is supported on the vehicle frame for swinging movement about a vertical axis. The backhoe is swingable from one side to the other of the vehicle about the vertical axis to facilitate the trenching, ditching or other digging operations. The swinging movement is actuated by a pair of pivotally supported hydraulic actuators extending between the vehicle frame structure and the mast or swing post.
One of the earliest patents disclosing a commercially acceptable system for actuating a swing post or mast mounted bucket is U.S. Pat. No. 3,047,171 issued to E. B. Long on July 31, 1962 for a "Swing Mechanism for Backshoe". One feature covered by this patent is that of providing a hydraulic "buffer" or cushioning arrangement that functions to decelerate the rotation or swing of the swing post or mast of the backhoe just prior to the end of its swing. This is accomplished by a dual hydraulic cylinder arrangement in which movement of one of the cylinders over center causes reverse movement of its piston creating a back pressure to check swinging of the swing post or mast. In order to absorb the pressure surge caused by such cushioning, the hydraulic system is provided with a relief valve, the excess pressure fluid being discharged ultimately into the fluid supply reservoir or tank. The hydraulic system also includes directional control valve means for routing fluid under pressure to one or the other of the cylinders, depending on the direction it is desired to swing the backhoe boom.
It apparently was the opinion of persons subsequently active in the development of backhoe control devices that more positive braking or cushioning devices than pressure relief valves were required to control the momentum forces of the heavy backhoe mast and swing units as the end of the swing movement was approached. Reference may be made to U.S. Pat. Nos. 3,630,120 and 3,815,766, both of which issued to Earnest E. Carlson et al. The U.S. Pat. No. 3,630,120 entitled "Swinging Apparatus" was issued Dec. 28, 1971; and the U.S. Pat. No. 3,815,766 entitled "Backhoe Swing Mechanism" was issued June 11, 1974. The Carlson et al patents disclose "sequence valve means for sensing the position of the hydraulic motors or actuators and for directing the flow of fluid to aid in obtaining a relatively constant torque output and angular velocity, bleeder valve means for reducing hydraulic input energy when the pivotal member (mast or swing post) is near the end of its rotation and for maintaining a maximum velocity potential throughout the swing movement, and relief valve means for additionally controlling torque output." The function of Carlson et al's array of valve means is to provide a positive braking or cushioning action not found in the earlier systems, but the result is a valve system believed to be of unnecessary complexity for the benefits derived.